As mobile phones and other digital electronic devices become increasingly smaller and thinner, the need for chips that are smaller yet offering larger capacitance is increasing every year for the multi-layer ceramic capacitors (MLCC) installed on the electronic circuit boards of these devices. In general, reducing the capacitor size inevitably reduces the area of the internal electrodes facing the dielectric layers, which in turn decreases the capacitance. To make sure the capacitor has sufficient capacitance to permit use of a smaller chip, high-density layering technology is crucial that reduces the thickness of the dielectric layers to be provided between the internal electrodes and also allows for layering of many of these dielectric layers.
To increase the density of dielectric layers in a multi-layer ceramic capacitor by making them thinner, one possible way is to minimize the grain size of the primary constituent of dielectrics, such as BaTiO3 (barium titanate). However, making the grain size of dielectrics finer to reduce the thickness of the dielectric layer causes the dielectric constant to drop due to the sizing effect, making it impossible for the capacitor as a whole to provide sufficient capacitance. In the field of high-density layering for ceramic capacitors, therefore, technology is adopted to sinter a fine powder of dielectrics to grow the grain size and thereby prevent the dielectric constant from dropping and consequently ensure sufficient capacitance.
According to Patent Literature 1, for example, a specific dielectric constant of 6000 or more can be achieved by adjusting to a range of 0.1 to 0.2 μm the grain size of the material powder, or specifically Ba1-xCaxTiO3 (also referred to as “BCT”) which is BaTiO3 partially substituted by Ca, and growing the dielectric grains to a grain size of 0.35 to 0.65 μm, in the process of reducing the thickness of the dielectric layer to approx. 1 μm. In general, Mg is added as an acceptor element to suppress the reduction of dielectrics due to sintering. In Patent Literature 1, the content of MgO in the dielectrics is 0.05 to 3.0 mol per 100 mol of Ba1-xCaxTiO3.
In addition, preferably the size of the dielectric grain is 200 nm or less in order to ensure sufficient electrical insulation property based on a dielectric layer of 1 μm or less. That is because the smaller the grain size, the larger the grain boundary becomes where movement of oxygen vacancy migration in the electrostatic field (electric field migration) is inhibited. If the grain size is large, on the other hand, the gaps between grains on the surface of the dielectric layer become deeper and the internal electrode paste will seep into these gaps to make it easier for electrode projections into the dielectric layer to form. A multi-layer ceramic capacitor is deemed equivalent to an electrical circuit comprised of individual capacitors, each constituting one dielectric layer, connected in parallel by the number of layers, and accordingly the entire capacitor will become electrically conductive if even a single dielectric layer is shorted due to concentration of electric field. Since this concentration of electric field occurring at electrode projections that generate in the gaps between grains can cause the insulation property to deteriorate and the voltage endurance to drop in the capacitor, it is desirable that the dielectric layer and interface contacted by the internal electrode be uniformly flat.
For example, Patent Literature 2 discloses a dielectric ceramic composition whose primary constituent is (Ba1-xCax)m(Ti1-YZrY)O3 (also referred to as “BCTZ”), average crystal grain size after sintering is 0.15 to 0.51 μm, and grain distribution based on the 100% equivalent dielectric grain size less the 50% equivalent size is 0.3 to 0.9 μm. The specific dielectric constant of the dielectric ceramic composition disclosed therein is 1651 or less.